The so-called polylaminated or polywoven material that is widely used to fabricate bulk material bags comprises a single ply material that has two layers intimately bonded together so that the two layers appear and function as a single ply. The inner layer is a polywoven scrim layer and the outer layer is typically paper or polypropylene. During manufacture of the raw polylaminated sheet, the polywoven scrim is inseparably and intimately bonded to the outer paper or polypropylene. A variety of materials may be used to make polylaminated sheets. The inner polywoven scrim is a fabric material that is typically woven from a polypropylene or high density polyethylene thread. As noted, the outer layer is typically a kraft paper, but also may be a bio-oriented polypropylene or an oriented polypropylene (also known as synthetic paper). Regardless of the particular materials that are used, the inner scrim layer is inseparable from the outer layer. Sometimes an extruded polyethylene layer is laid down between the inner scrim layer and outer paper layer.
The raw polywoven sheet is formed into bulk material bags that are filled by the consumer. A variety of bulk material bags may be formed, but typically the bags are of the “sewn open mouth” type. This type of a bag may or may not have side gussets, but in either case one end of the bag is typically sewn closed and the bag is shipped from the manufacturer to the user empty, in a flattened condition. The user fills the bag with bulk material such as pet food, agricultural commodities, chemicals and the like through the open top, and the open top is then sewn closed by the consumer, resulting in an efficient bag that is generally suitably strong.
While sewn closures on both ends of a sewn open mouth polywoven bag are an industry standard, such closures have inherent limitations. For example, sewn closures inherently introduce holes in the bag. Holes can be a problem because they present an entry route for insects and other contaminants. Insect contamination in bulk bags containing pet food is a notorious problem. Just as well, other contaminants are able to enter the interior of a bag through the holes sewn through the bag in sewn closures. A folded-over closure is not an option with current sewn open mouth polywoven bags because the open top of the bag has only exposed paper on the outer layer. If the open end is folded over there is no material that could be bonded together. While a stepped end could be a possible solution to this problem with raw materials other than polywoven, a stepped open end is not an option with polywoven material because the way in which the raw sheets are manufactured. It is possible for a user to melt the polywoven material together in a heated band sealer, but this type of closure requires specialized equipment that most users do not have access to, and may not be an adequate solution where the material in the bag is powdery and powder covers the polywoven material. Moreover, sealing a polywoven bag with heat is not an ideal closure because it is difficult to control the amount of melted material that forms the seal, and there may be leaks and or overheating, which could lead to cracks. Finally, while sewn closures have adequate strength for most uses, the pinch bottom closures provide a seal without sifting.
Accordingly, in view of the shortcomings inherent in conventional polywoven bags, there is an opportunity to supply a bag that has all of the advantages of conventional bags, yet avoids the shortcomings.